A filter element for filtering MM optical signals may be a multilayer thin-film device that relies on the interference of multiple reflected beams from a stack of quarter-wave dielectric layers of alternate high and low refractive index. Incident light undergoes multiple reflections between the coated layers which define the cavity. When the beams reflected from all the interfaces in the assembly are of equal phase at the front surface they combine constructively, i.e., whenever there is no phase difference between the emerging wavefronts, interference produces a transmission maximum.
A typical prior art filter construction for a narrow (1 nm) bandpass filter is shown in FIG. 1. This filter consists of two or three cavities back-to-back to provide steeper band slope, improved near band rejection, and "square" passband peaks. Each cavity consists of 21 layers of alternate high and low index material. These may be soft coating material such as zinc sulfide ZnS and cryolite Na.sub.3 AlF.sub.6 or hard coating material such as Ta.sub.2 O.sub.5 and SiO.sub.2.
To use an interference filter to filter a signal from a fiber, the fiber output should be collimated. This is accomplished by using a beam expander device which may be constructed with either aspheric or GRIN (Graded Index) lenses as shown in FIG. 2 and inserting the interference filter element in the collimated beam. In the case of a SM (single mode) input signal the collimation is very effective and the interference filter behaves as though the source is perfectly collimated. The situation for a MM source which is not perfectly collimated is considerably more complex. Due to a number of modes in the MM fiber a single lens element cannot perfectly collimate the fiber output. This results in the beam having a distribution of angles for any filter position.
It should be noted that since the output from a MM fiber source is not perfectly collimated it is important to minimize the separation distance between the collimating elements to avoid excessive signal loss in the beam expander device (see FIG. 2).
When using an interference filter to select a Wavelength Division Multiplexed (WDM) channel, the filter must be tunable over a given WDM wavelength range. Even though the interference filter element is designed for a given passband and a given wavelength for incident radiation at normal incidence, the interference filter passband may be tuned to shorter wavelengths by tilting the filter with respect to the incident radiation. When the tilt angle is small the major effect is in the phase thickness of the layers, which is affected equally for each plane of polarization. For larger tilts the admittances are also affected and then the performance for each plane of polarization differs.
When an interference filter is tuned by tilting the filter there is an associated loss penalty which depends on the degree of collimation of the incident radiation. The biggest limitation to use of the Bandpass (BP) Interference filter to filter MM WDM signals from a fiber is its excess loss due to tuning over the desired wavelength range of 1548 to 1560 nm (for Erbiun Doped Optically Amplified Systems). The BP filter tuning loss is much greater for a MM configuration because the MM beam is not perfectly collimated and the interference filters performance is optimum for incident plane waves. If non-parallel light is incident on a filter the peak transmittance is reduced, the bandpass is broadened, the center wavelength is shifted, and the spectrum becomes asymmetric.
FIG. 3 shows the filter angle and total induced loss as a function of tuning away from the center wavelength. The filter input is a MM source with a 100/140 .mu.m MM fiber on a WDM system. The filter uses an aspheric type beam expander and dual cavity 1 nm BP filter element with a center wavelength of 1550 nm. The gap spacing in the beam expander, i.e., the distance between lens elements, is preferably set to 12.5 mm.
The excess tuning loss may be minimized by reducing the wavelength range over which the filter must be tuned. This may be accomplished by providing multiple filters to limit the range over which each filter element must be tuned as in the present invention.